This invention relates to fumed silica and more particularly it relates to a method of producing fumed silica and other products.
Fumed silica is used as a thickener or reinforcing agent in inks, resins, rubber, paints and cosmetics, for example. There are many processes in the art which disclose the production of silica, or fumed silica, or which disclose processes for the purification of silica. For example, U.S. Pat. No. 4,559,218 discloses a process and apparatus for preparing finely divided silicon dioxide having high thickening capacity and good thixotropic properties by reaction of silicon fluoride in the vapor phase with water vapor, combustible gas and free oxygen-containing gas in a flame reaction zone to form silicon dioxide and hydrogen fluoride while cooling the gaseous reaction mixture in that portion of the reaction zone adjacent the base of the flame by contact with a cooling surface maintained at a temperature below 500.degree. C. but above the dew point of the reaction waste gases generated in the flame reaction.
U.S. Pat. No. 5,853,685 discloses a process for the production of high purity silica from waste by-product silica and hydrogen fluoride. In this process high purity silica is obtained by the reaction of impure by-product waste silica with hydrogen fluoride preferably in the presence of water or sulfuric acid, producing silicon tetrafluoride gas and a mother liquor. The silicon tetrafluoride is separated from the mother liquor, which retains the impurities originally contained within the impure silica. The silicon tetrafluoride gas is contacted with high-purity water, in a clean environment, to form a slurry of high purity silica and high-purity hydrofluosilicic acid (FSA). A portion of the silica is filtered from the slurry and washed producing a high purity silica product. The rest of the silica-FSA slurry is preferably reacted with ammonia to form a slurry of ammonium fluoride and silica. The silica is separated from the ammonium fluoride and preferably washed and calcined to remove any remaining ammonium fluoride, leaving additional high purity silica product.
U.S. Pat. No. 5,679,315 discloses a method for production of high purity silica and ammonium fluoride from silicon tetrafluoride-containing gas, particularly the gas generated by acidulation. The basic method comprises recovering silicon tetrafluoride-containing gas from the acidulation of a fluorine-containing phosphorus source, separating liquid entrainment from the gas, converting the gas recovered to an ammonium fluosilicate solution, and ammoniating said ammonium fluosilicate solution to produce high purity silica and ammonium fluoride.
U.S. Pat. No. 2,780,523 discloses a process for recovering solid calcium fluoride containing product and colloidal silica solution from a fluosilicic acid solution. This process uses calcium carbonate to react with the fluosilicic acid.
U.S. Pat. No. 4,308,244 discloses a process for producing fluorine compounds and amorphous silica in which hydrofluoric acid and/or other fluorine compounds are recovered by reacting fluosilicic acid with a sodium-containing compound to form an alkaline aqueous slurry comprising precipitated amorphous silica and dissolved sodium fluoride. The precipitated amorphous silica is separated from the alkaline aqueous slurry leaving an aqueous solution of sodium fluoride. The sodium fluoride solution can be used as such or reacted with other substances (such as alumina, to form synthetic cryolite). Sodium fluoride is recovered from the aqueous solution (as by evaporation or extraction) and used per se or reacted with sulfuric acid to produce hydrogen fluoride.
U.S. Pat. No. 4,508,538 discloses a method of recovering fumed hydrophobic SiO.sub.2, produced by the flame hydrolysis of a ternary azeotrope of H.sub.2 SiF.sub.6, HF, and H.sub.2 O. The method involves passing the fumed hydrophobic silica, as it leaves the combustion chamber, through a nonclassifying agglomerator at a residence (travel) time of from about 0.5 to several seconds, preferably about 1.0 to about 5.0 seconds, then separating the filmed silica from the product stream in a cyclone separator or other means for collecting dispersoids.
U.S. Pat. No. 4,572,827 discloses a process for preparing finely divided silicon dioxide in good yield by reaction of silicon fluoride in the vapor phase with water vapor, combustible gas and free oxygen-containing gas in a flame reaction zone to form hydrogen fluoride and silicon dioxide entrained in a gaseous reaction mixture; withdrawing the gaseous reaction mixture from the flame reaction zone; and then immediately and rapidly cooling the gaseous reaction mixture and entrained silicon dioxide to a temperature below 700.degree. C. by passing the gaseous reaction mixture in a turbulent flow at a Reynolds number above 300 under constraint through a straight narrow passage in alignment with the gas flow, the passage having a diameter within the range from about 20 to about 150 mm and smooth walls constituting a cooling surface.
U.S. Pat. No. 4,915,705 discloses a process for producing high grade silica and fluorine-containing coproducts from fluosilicic acid. To produce silica, the fluosilicic acid is reacted with aqueous ammonia to produce a solid silica product and a solution of ammonium fluoride. The solid silica is washed under controlled conditions and further processed to produce a high grade product. The ammonium fluoride solution is concentrated and sold as a concentrated ammonium fluoride solution product or reacted with metal hydroxides or oxides to produce metal fluorides.
U.S. Pat. Nos. 4,981,664 and 5,165,907 disclose a method for producing high purity silica and ammonium fluoride from silicon tetrafluoride-containing gas wherein silicon tetrafluoride-containing gas from the acidulation of phosphorus-containing rock is recovered and the liquid entrainment is separated from the gas. The recovered gas is converted to an ammonium fluosilicate solution and is ammoniated to produce high purity silica and ammonium fluoride. The recovered gas can be converted to an ammonium fluosilicate solution by absorbing the gas directly in a solution of ammonium fluoride.
U.S. Pat. Nos. 5,266,289 and 5,458,864 disclose a process for producing a high-purity silica, which comprises the steps of reacting a crude silica with ammonium fluoride, acid ammonium fluoride or a mixture thereof in an aqueous medium to produce ammonium silicofluoride, separating the ammonium silicofluoride from an unreacted silica and impurities by the means of a solid/liquid separation, and reacting the thus-obtained ammonium silicofluoride with ammonia in an aqueous medium. This process is a low cost process, and enables to control properties of silica.
U.S. Pat. No. 5,340,560 discloses a method for making fumed silica aggregate having an average convex perimeter in the range of about 0.12 micron to about 0.6 micron based on the combustion of a mixture of a silicon compound, such as an organosilane, an oxygen containing gas, such as air and a fuel such as hydrogen. The fumed silica aggregate has been found to enhance properties in heat curable filled silicone
U.S. Pat. No. 5,428,546 discloses a process for preparing powder of silicon compound wherein an alkoxysilane is brought into a hydrolysis-polycondensation reaction using a fluoride salt, e.g., NaF, or a fluorine-containing silicon compound having an Si--F bond in its molecule, e.g., FSI(OCH.sub.3).sub.3, as a catalyst, to produce a powder of a silicon compound such as silica. The process reduces markedly the period of time required for the hydrolysis-polycondensation reaction and yields a porous silicon compound powder with low bulk specific gravity.
In spite of these references, there is still a great need for an economical process for preparing fumed silica and ancillary end products. The present invention provides such a process.